Voltage dependent resistor



April 5, 1966 w HEYWANG ETAL 3,245,019

VOLTAGE DEPENDENT RESISTOR 3 Sheets-Sheet 1 Filed Oct. 23, 1964 Fig.1a

April 5, 1966 w. HEYWANG ETAL, 3,245,019

VOLTAGE DEPENDENT RESISTOR VOLTAGE DEFENDENT RESISTOR 3 Sheets-Sheet 5Filed Oct. 25, 1964 Fig.4 5-10 judo/f 5024 6 fa United States Patentmany Filed Oct. 23, 1964, Ser. No. 406,141 Claims priority, applicationGermany, June 4, 1958,

S 58 485 9 Claims. (in. ass-20 This application is a continuation inpart of copending application Serial No. 816,994, filed May 29, 1959.

The invention relates to resistors with high positive temperaturecoetficient of the resistance value over a range of temperatures, havingthe following features, namely: (a) the resistor is made of a ceramicferroelectrical material whose Curie temperature, at which the materialloses its permanent polarization, lies at least below the upper limit ofthe operating temperature range, especially at or below the lower limitof the range at which the resistor should have a positive temperaturecoefiicient of the resistance value, and preferably below 20 C.; (b) theresistor material is made conductive by the inclusion of impuritycenters by donor or acceptor (preferably n-conductive whereby therespective spacing E between the energy level of the donor atoms and theconduction band, or between the energy level of the acceptor atoms andthe valence band, is smaller, and especially appreciably smaller, thanhalf of the width E of the prohibited zone between valence band andconduction band; and (c) the intrinsic conductivity or another impuritycenter conductivity of the resistor material not imparted by said donoror acceptor atoms is slight, at least in part of the operatingtemperature range in which the resistor has a positive temperaturecoefiicient, and especially negligibly slight as compared with that ofthe impurity center conductivity.

It has further been proposed, according to copending application SerialNo. 809,478, filed April 28, 1959, now Patent No. 3,027,529, also ownedby the same assignee, to dispose the contacts substantially free ofbarrier or blocking layers, so as to obtain in such a resistornegligibly slight voltage dependence of the total resistance. It wasfound, however, that a voltage dependence is despite these measuresfrequently present at sufiiciently high field strength. In order topermit operation of the resistor with high voltage without practicallyproducing a voltage dependence of the resistance value, this copendingapplication therefore suggests to provide for predetermined grain orparticle sizes of the resistor material, which have the effect that theresistance remains up to praticularly high field strength practicallyvoltage independent.

The present invention is directed to a voltage-dependent ceramicresistor of the type disclosed in said Patent No. 3,027,529, wherein theconcentration of the donor or acceptor atoms (n in the interior of theparticle and the particle size d are such that the magnitude of theproduct 61,71 is so much larger or smaller than that magnitude of theproduct dJ'I at which a maximum value (E for the critical field strength(E would be produced with any surface state density (11 that the actualvalue of the critical field strength is less than half of said maximumvalue (see FIG. 1a).

In order to obtain a resistor which is voltage dependent While being astemperature independent as possible, the present invention proposes toplace the Curie temperature of the resistor material with respect to theoperating temperature for which the resistor is designed, so that thevalue of the resistance is at the corresponding operating voltages to ahigh degree temperature independent while being strongly voltagedependent. This invention 3,245,019 Patented Apr. 5, 1966 is based onthe concept, explained in the above referred to patent that potentialwalls are formed at the particle borders (see FIG. 3 of such patent) dueto surface impurity terms, which are in the presence of sufficientlyhigh field strength, resulting in the resistor material upon connectionof the operating voltage, overcome thermally by the electrons flowing inthe conducitivity band of the resistor material.

The invention and the various features and objects thereof will now beexplained with reference to the accompanying drawings, wherein: 7 FIG.1, which corresponds to FIG. 3 in said Patent No. 3,027,529, shows theeffect of the particle size on the field strength, producing a criticalfield strength 13,, at which resistance value begins to break down;

FIG. 1a shows the critical field strength E plotted against the factorsof the particle size;

FIG.2 shows an example of a resistor in which the invention may beincorporated; and

FIGS. 3 and 4 are performance curves.

FIG. 1, corresponding to FIG. 3 of said patent, shows in schematicmanner an n-doped titanate and the formation of the previously mentionedpotential wall. The vertical line 1 indicates the particle border, forexample, of two intersintered barium titanate bodies. Numerals 2 and 2"represent donor terms of two particles I and II lying closely to thelower border 3' and 3" of the conductivity band of the barium titanate.The upper limit of the valence band of those titanates is indicated at4' and 4". At the particle border 1 there is at A an acceptor termwhich, due to its energetically low position leads to a discharge of theneighboring donors 21', 21, such donors accordingly forming a spacecharge zone and tending to bend the conductivity curve upwardly. Theheight of this bend is indicated by (p thus designating the height ofthe potential wall which is now within the range of the Curietemperature due to the very strong temperature dependence of thedielectric constant (e), and is to a high degree temperature dependent.It rises, for example, in the case of barium titanate at an assumedacceptor density at the particle border, of l0 l0 /cm. and at a donordensity of about 10 -10 /cm. from about 0.1 volt at 20 C. to about 1volt at- 200 C.

A low voltage dependence of the resistivity value occurs when thepotential wall is by the electrons flowing in the conduction bandsubstantially not overcome by the wavemechanical tunnel effect but onlythermally; however, in order to achieve this, the maximum field strength(E g) at which the resistor is operated, must not be appreciablygreater, at the particle borders, even at the lowest temperatures, thanthe quotient formed by the height to of the potential wall divided bythe particles size d. I The value E /d is the mean critical fieldstrength 13,; in the volume of the resistance material at which theresistance value caused by the potential wall at the particlesboundaries starts to collapse height of the potential wall, see FIG. 1,d length of particle measured in the direction of the working fieldstrength in the volume of the resistance material hereinafter referredto as particle size. The mean critical field strength 15;, is given bythe formula:

depending on whether the particle size a is larger or smaller than n /nIn these formulae 11 is the surface state density (number of acceptor ofdonor atoms for unit area of the particle boundaries), m the impuritycenter concentration (total number of donor or acceptor atoms for unitvolume of the particles), s the dielectric constant of a vacuum, and eis the dielectric constant of the resistance material. In any particularceramic the surface state density n is a constant and, if (see FIG. 1aof the present application) the magnitude of the mean critical fieldstrength E of the potential wall is plotted against the product n -d fora particular ceramic the curve shown in FIG. 1a is obtained. So that thecollapse of the resistance occurs at critical field strengths E; whichare less than half of the maximum value, the particle size d must eitherbe kept below the magnitude which corresponds to half the maximum fieldstrength for a given density of the impurities (11 embedded in theresistance material, or else must be made so large that the product n -dis so much greater than that corresponding to the broken line in FIG. 1athat the critical field strength falls to less than half its maximumvalue. In one particular ceramic with a convenient impurity density ithas been found advisable to make the particle diameter of the majorportion of this ceramic voltage-dependent resistor either less thanmicrons and preferably less than 2 microns, or else greater than 15microns and preferably between 20 and and 30 microns.

FIG. 3 hereof is a graph illustrating the dependence of the resistanceof the resistor at very low voltages on the temperature. It will be seenthat well below the Curie temperature which is indicated by an arrow 11the resist ance is at a low value which is even below approximately 100ohm-cm. At higher temperatures, the resistance rises to nearly ohm-cm.and has therefore, increased by several powers of ten. In order toobtain in such a resistor according to the invention a resistance valuewhich varies with temperature as little as possible, the resistancevalue of the resistor body measured at a low voltage of the Workingvoltage range and at low temperatures of the working temperature rangeshould be large compared with the resistance value of the same resistormeasured at the same voltage but at the Curie temperature or at lowertemperatures. In FIG. 3 arrow b indicates the lower limit of the workingtemperature range at which a resistor according to the invention inwhich the ferroelectric substance is barium titanate operates as avoltage-dependent resistor and has only a small or no temperaturedependence. This temperature b is between 50 and 100 above the Curietemperature indicated by the arrow a. If, therefore, the lowest workingtemperature at which the resistor is required to serve as a varistorindependent of temperature lies near room temperature (approximately 20C.) it is advisable to use as starting material a material the Curietemperature of which lies below 20 C., and preferably below 0.

When a high voltage dependence of the resistance is desirable, astarting material is advisable whose Curie temperature lies more than100 below the lowest limit of the working temperature range; sometemperature dependence of the resistance value must then be tolerated,see for instance FIG. 3 in which above approximately 300 C. theresistance begins to drop again in dependence on the temperature. If,however, as high a temperature in dependence as possible is ofimportance, it is advisable to operate the resistor on which FIG. 3 isbased bebetween the temperatures b and 0 (approximately between 180 and340 0.). It is, therefore, advisable to choose the material of theresistor for a given working temperature range to be such that themaximum resistance value measured at low voltages of the working voltagerange lies within the working temperature range. In the case illustratedin FIG. 3, the maximum resistance value lies at point M, that is at atemperature at approximately 250 C. which is between the limits b and cof the working temperature range.

FIG. 4 is a graph illustrating the dependence of the resistance of theresistor, the R-T characteristic of which is illustrated in FIG. 3against the field strength in the resistor for various temperatures. Asmay be seen the resistance is strongly voltage-dependent at fieldstrengths above approximately 10 v./cm. In particular it is advisable todimension the resistor in such a way that the working voltage range hasa lower limit corresponding to a field strength of approximately v./cm.In this way the resistor is made strongly dependent on the appliedvoltage (see the larger slope of the curves in FIG. 4).

The measurements of FIGS. 3 and 4 are based on a resistor of the typespecified, the starting material of which is pure barium titanate.Consequently the Curie temperature lies at approximately C. and theworking temperature at which this resistor is to be operated as avaristor, that is with a strong voltage dependence but with a smalltemperature dependence, lies above approximately C. It is possible toshift the Curie temperature and, therefore, the steeply rising branch ofthe resistance curve to corresponding low temperatures in those cases inwhich other working temperature ranges are desirable such as, forinstance, a temperature range between 0 and 100 C. For this purpose adifferent ferro-electric material with a considerably lower Curie pointmay be used as a starting material for a resistor according to theinvention. Preferably, a barium strontium titanate may be used in whichthe proportion of the strontium in the titanate amounts to 30 to 50 molpercent or more of the titanium in the titanate. In particular the Curietemperature of the starting material may lie at or below 0 C., forinstance at approximately -20 C., for a working temperature rangeextending from approximately 0 C. or room temperature up toapproximately 100 C. As mentioned above, it is, however, advisable tolocate the Curie temperature for this working temperature range stilllower, in particular at between 50 and -l00 C.

It was already explained with respect to FIG. la it is in generaladvisable, in order to initiate the collapse of the resistance independence on the applied voltage at low field strengths, to work in theleft hand portion of the linear branch of FIG. 1a, that is at as small agrain diameter d as possible. The resistance material should, therefore,be sintered from particles which are in general smaller than 5 micronsand preferably smaller than 2 microns. However, the critical fieldstrength E is inversely proportional to the abscissa d-n in the righthand portion of FIG. 1a. The curve extends therefore hyperbolically tothe right of the broken line in FIG. 1a. Since the magnitude nfrequently cannot be made as small as desired the particle diameter dmust often be made undesirably small in order to arrive at suflicientlylow value B In these cases it is advantageous to make the particlediameter particularly large, that is to make it so large that theproduct n -d is greater than that corresponding to the broken line inFIG. 1a. The critical field strength E; at which the resistance of theceramic body starts to collapse and its resistance value becomes,therefore, strongly voltage-dependent is then achieved at relatively lowvalues of the voltage applied to the resistor. In particular, it is thenadvisable to make the particle diameter d greater than approximately 15microns, preferably approximately equal to 20 to 30 microns or more. Inthese cases too, however, as stated'in said Patent No. 3,027,529 theparticles preferably are to be used which are as uniform as possible sothat the particle sizes of the major portion of the ceramic resistancematerial deviate relatively little, that is by only approximately :L-20to 25% from their mean value in this body.

The invention may be applied in the construction of resistors of thetype as shown, for example, in FIGS. 1 and 2 of said patent, one suchembodiment being shown herein in FIG. 2.

As explained in said patent, the contacts are placed upon the resistorbody substantially free of barrier or blocking layer, especiallyvaporized thereon, so as to reduce the voltage dependence of the totalresistance value of the resistor or to make it negligibly low at roomtemperature. The material for the contacts is for this purpose selectedso that it does not form a barrier or blocking layer with the resistormaterial; more particularly, in the case of aresistor sintered offerroelectric crystallite particles made n-conductive, the currentsupply contacts will consist of a base metal, preferably aluminum orzinc or of an alloy containing at least a high propor tion of one ofthese metals. The surface parts of the ceramic material serving for thecontacting are more- :over prior to or if desired incident to thecontacting preferably particularly treated as compared with the interiorparticles of the resistor body, especially mechanically treated, forexample, they are made to be 'well conductive by solder rubbing or bysanding or by electrical or chemical treatment applied prior tovaporizing metal thereon. The purpose of this pretreatment is to providea clean surface free of troublesome terms. This may be obtained, forexample, by glow treatment or by chemical reduction. The treatment maybe such as to affect not only the surface but penetrating to some depththe marginal layer of the resistor material.

In order to keep as small as possible the varistorlike volume effectcaused by the transition resistances between the crystallites of thesintered resistor body, it is furthermore proposed that the resistormaterial be sintered at suitable high temperature for a sufiicient timeso as to avoid these transition resistances as far as possible. In theevent that an impermissible voltage dependence of the specificresistance cannot be avoided in the production of the resistor body, bysuitable selection of the sintering conditions, the volume effect may besubsequently reduced by forming operations.

For example, as explained in said patent, a suflicientlyvoltage-independent resistance may be produced by first sintering theresistor body without regard to the voltage dependence of itsresistance, thereupon contacting the resistor body in the mannerexplained above, and thereafter passing through the resistor a strongcurrent surge which reduces the voltage dependence of the transitionresistances between the crystallites and therewith the conductivity ofthe resistor body to a tolerable point, by effecting in a mannerweldingtogether of the individual crystallites.

Since a reduction of the volume resistance could in connection with thetreatment he observed, if at all, only at high field strength, while theresistance remained at lower field strength practically independent ofthe voltage applied, it is possible by providing suitable dimensions ofthe resistor body to produce for any given voltage a thermistor with anydesired resistance value, which is practically free of varistor effects,by making the spacing between the two resistor contacts relativelygreat, therewith the field strength low, and making the cross-sectionalarea fo the resistor body correspond to the desired capacitance and thelength of the current path in the resistor body.

The surface treatment is in a particularly advantageous embodimentcarried out by subjecting the surface to a glow effect. For thispurpose, the semi-conductor is suitably disposed in a vacuum vessel, atrelatively low gas pressure, opposite an electrode and the surface partsof the semi-conductor body which are subsequently to be contacted arebrought to a glowing condition by the connection of an alternatingvoltage or, preferably, a direct voltage, which becomes effectivebetween the resistor body and the electrode. When using a directvoltage, it is advisable to place the semi-conductor on the positivepole of the voltage source. The resistor material may be subjected to astrong glow effect, for example, at a current density from about to 30milliamperes per square centimeter, for example, at roughly 3000 to 5000volt, so as to liberate the surface to be metallized also from adheringresidual gas or other contaminations such as deposited hydrogen. Thecontact metal which does not form a blocking layer with thesemi-conductor is thereupon applied, advantageously by vaporization inthe same vacuum vessel at further reduced gas pressure The con- 6 tactmetals to be used in the case of n-doped ferroelec'tric resistormaterials are preferably base metals with a normal potential below thatof silver, especially below that of copper, for example Al or Zn, whichare particularly suitable. The use of noble metals as contact materialsis, however, not inherently excluded.

The current connections may be mechanically secured on thesemi-conductor resistor, for example, in the case of rod-shapedresistors (not shown herein), in the form of caps attached thereto bypress-fit or in the form of clasps embracing the resistor body.

FIG. 2 shows a disc or wafer-shaped resistor comprising a resistor body1 with a width L. The resistor body 1' is made of sintered ferroelectriccrystallites. Its opposite sides 11 and 12 are pro-treated as descnibedbefore, for example, chemically or electrically to remove surface terms.Instead of providing current connections in the form of caps or thelike, as in the case of a rod-shaped body, base metal, for example,aluminum 13, 14 is placed upon the resistor body 1, preferably byvaporizing. These vaporized layers or coatings 13, 14 form with thesurfaces 11, 12 of the ceramic resistor body 1 very good andsubstantially barrier-free contact. The base metal coatings arethereupon provided, chemically or electrochemically, With solderablematerial, for example, silver, forming thin reinforcing layers 15, 16,to which are soldered disc shaped portions 171, 181 carried by therespective current terminals 17 and 18, numerals 19 and 20 indicatingthe resulting solder pearls.

Changes may be made within the scope and spirit of the appended claimswhich define what is believed to be new and desired to have protected byLetters Patent.

We claim:

1. Ceramic resistor exhibiting strong voltage dependence of itsresistance value, said resistor being made of ferroelectric-al materialhaving a Curie temperature, above which the material loses its permanentpolarization, lying below the lower limit of the range at which theresistor shall have said positive temperature coefl'icient of theresistance value, said resistor having conductive impurity content witha spacing of the donors from the conduction band and of the acceptorsfrom the valence band which is smaller than one-half of the width of theprohibited zone between the valence band and the conduction band, theintrinsic conductivity of the resistor material being, at least withinthe range of the positive temperature coeificient, small as comparedwith the conductivity of said impurity content centers, the resistancevalue of said resistor, measured at a low voltage of the voltage rangeand at low temperatures of the operating temperature range, being highas compared with the resistance value measured at identical voltage butat a temperature not exceeding the Curie temperature of the basicresistor material, the concentration of the impurity atoms (n in theinterior of the particles and the particle size (0.) being such that themagnitude of the product (d 1%) difiers so greatly from that magnitudeof the product d -n at which a maximum value (E for the critical fieldstrength (B would be produced with any surface state density (n that theactual value of the critical strength is less than half of said maximumvalue.

2. A resistor according to claim 1, wherein the Curie temperature of theresistor material lies about 50 C. to over C. below the lower limit ofthe operating temperature range.

3. A resistor according to claim 1, wherein the Curie temperature liesat about 20 C. to 0 C.

4. A resistor according to claim 1, wherein the diameter of theparticles of the resistor material, measured in the direction of theoperating field strength and the main part of the resistance material,is smaller than 5 microns.

5. A resistor according to claim 1, wherein the diameter of theparticles of the resistor material, measured in the direction of theoperating field strength and the main part of the resistance material,is smaller than 2 microns.

6. A resistor according to claim 1, wherein the diameter of theparticles of the resistor material, measured in the direction of theoperating field strength and the main part of the resistance materialexceeds 15 microns.

7. A resistor according to claim 1, wherein the diameter of theparticles of the resistor material, measured in the direction of theoperating field strength and the main part of the resistance material isfrom about 20 microns to about 30 microns.

8. A resistor according to claim 1, wherein the particle 10 sizes in thematerial of the preponderant part of said resistor deviate by less than25% from the average particle size therein.

9. A resistor according to claim 1, wherein theresistor body is diskshaped, contacts of base metal disposed barrier-free on said body, saidcontacts reinforced by layers of solderable metal disposed thereon.

References Cited by the Examiner UNITED STATES PATENTS 2,864,713 12/1958Lewis 106-39 2,911,370 11/1959 Kulcsar 25262.9

RICHARD M. WOOD, Primary Examiner. W. D. BROOKS, Assistant Examiner.

1. CERAMIC RESISTOR EXHIBITING STRONG VOLTAGE DEPENDENCE OF ITSRESISTANCE VALUE, SAID RESISTOR BEING MADE OF FERROELECTRICAL MATERIALHAVING A CURIE TEMPERATURE, ABOVE WHICH THE MATERIAL LOSES ITS PERMANENTPOLARIZATION, LYING BELOW THE LOWR LIMIT OF THE RANGE AT WHICH THERESISTOR SHALL HAVE SAID POSITIVE TEMPERATURE COEFFICIENT OF THERESISTANCE VALUE, SAID RESISTOR HAVING CONDUCTIVE IMPURITY CONTENT WITHA SPACING OF THE DONORS FROM THE CONDUCTION BAND AND OF THE ACCEPTORSFROM THE VALENCE BAND WHICH IS SMALLER THAN ONE-HALF OF THE WIDTH OF THEPROHIBITED ZONE BETWEEN THE VALENCE BAND AND THE CONDUCTION BAND, THEINTRINSIC CONDUCTIVITY OF THE RESISTOR MATERIAL BEING, AT LEAST WITHINTHE RANGE OF THE POSITIVE TEMPERATURE COEFFICIENT, SMALL AS COMPAREDWITH THE CONDUCIVITY OF SAID IMPURITY CONTENT CENTERS, THE RESISTANCEVALUE OF SAID RESISTOR, MEASURED AT A LOW VOLTAGE OF THE VOLTAGE RANGEAND AT LOW TEMPERATURES OF THE OPERATING TEMPERATURE RANGE, BEING HIGHAS COMPARED WITH THE RESISTANCE VALUE MEASURED AT IDENTICAL VOLTAGE BUTAT A TEMPERATURE NOT EXCEEDING THE CURIE TEMPERATURE OF THE BASICRESISTOR MATERIAL, THE CONCENTRATION OF THE IMPURITY ATOMS (ND) IN THEINTERIOR OF THE PARTICLES AND THE PARTICLE SIZE (D) BEING SUCH THAT THEMAGNITUDE OF THE PRODUCT (D.ND) DIFFERS SO GREATLY FROM THAT MAGNITUDEOF THE PRODUCT D.ND AT WHICH A MAXIMUM VALUE (EK MAX) FOR THE CRITICALFIELD STRENGTH (EK) WOULD BE PRODUCED WITH ANY SURFACE STATE DENSITY(NA) THAT THE ACTUAL VALUE OF THE CRITICAL STRENGTH IS LESS THAN HALF OFSAID MAXIMUM VALUE.